1. Field of the Invention
The present invention relates generally to electron-optic distortion phenomena in color CRTs. The present invention relates specifically to correction of an electron-optic distortion phenomenon resulting from use of a self-convergent (SC) deflection yoke having pin-cushion correction built into the yoke.
2. Discussion of the Related Art
In the art of color cathode ray tubes (CRTs) there are many known electron-optic distortions resulting from the imperfect deflection of the red, green, and blue electron beams as they are scanned across the phosphor screen by the deflection yoke. One such distortion particularly prevalent in CRTs having flattened or large radius faceplates is pincushion distortion where the four sides of the raster are curved inward. An example of such a CRT is the flat tension mask (FTM) computer monitor tube made by the assignee hereof.
Pincushion distortion can be corrected by proper distribution of the windings of the deflection yoke. However with reference to FIG. 1 this built-in correction causes a color purity distorting effect in a triad of electron beamlets passing through a common aperture in the shadow mask. This particular distortion is herein called "eyebrow effect". "Eyebrow effect" is the name given to the situation where the three in-line beamlets corresponding to red 29, green 32, and blue 33, designed to land substantially at a constant Y axis co-ordinate on the screen, instead exhibit a curved landing pattern with only the green beamlet 32 at the design coordinate and the red 29 and blue 33 beamlets lying in an arc above and below the desired Y-coordinate. The eyebrow effect is a dynamic effect, being largest in the corner regions and varying antisymetrically across the raster. It comes about because a pin cushion corrected yoke changes beam entry angles onto the mask. Color purity of the display is then compromised.
As seen in FIG. 2 the electron gun 11 located in the neck 13 of the CRT 15 emits three beams 17 coplanar in the X-Z plane. The beams 17 travel through a deflection yoke 19 where they are deflected to sweep across the screen 21. The beams hit a perforate parallax barrier or shadow mask 23, which passes a beamlet through the proper mask hole 25 to register the beamlet on the phosphor deposits (not shown) of the proper color luminescence to produce the desired display.
However, when a selfconverging yoke 19 has pin cushion distortion correction built-in, the yoke magnetic field displaces the outer beams, e.g. 47, changing their angle of incidence onto the mask so that they no longer arrive at the screen 21 on the same Y-axis coordinate as the center beam 51. This results in beamlet misregistration on the desired phosphor dot, causing the problems of color purity on the display seen in FIG. 1.
An unrelated electron-optic distortion called "rotation" or "twist" is caused by accidentally mounting an in-line three beam electron gun so that it is rotated about the tube axis within the CRT neck, or by the gun itself emitting "mis-aimed" beams. Rotation is a static phenomenon that gives rise to beam misconvergence on the face plate. Normally, such gun rotation would be corrected by a matching rotation of the yoke. This solution, however, rotates the raster with respect to the display unit escutcheon. U.S. Pat. No. 5,117,151, issued May 26, 1992 to Sluyterman et al., teaches the placing of permanent magnets around the neck of the CRT between the gun and the yoke to counterbalance the static rotation of the beams emitted from the gun before the beams reach the yoke, thus reducing or eliminating beam misconvergence without rotating the yoke.
The magnets of Sluyterman et al. deflect the displaced beams twice, so as to place them into the positions they would have occupied had the gun not been rotated. Therefore, the beams are properly placed within the yoke field, enabling the yoke to perform according to its design and to converge the beams properly.
In the eyebrow effect, the beams may remain converged on the screen, but through an unintended side effect of the pincushion correction built into the yoke windings, the yoke field displaces the outside, or red and blue, beam paths from their normal angle of incidence onto the mask, thereby misdirecting the beamlets which form as the beams pass through the mask holes, and misregistering the areas of impact of the beamlets on the phosphor deposits. As a result, color purity is compromised. Physical location of the phosphor dots could be changed to correspond to beamlet landings, yielding a small amount of eyebrow correction limited by the concurrent reduction of the phosphor dot guard bands.
It is an object of the present invention to dynamically correct eyebrow effect distortion by dynamic pre-yoke beam deflection. It is also an object of the present invention to perform this correction while maintaining beam convergence. The present invention has magnetic field generators placed between the gun and the yoke and energized synchronously with the beam scan to cancel the eyebrow effect distortion by dynamically predeflecting the beams so as to counterbalance the eyebrow effect induced by the yoke.